This invention relates to apparatus and methods for opening perforations in well casings, and more particularly, to a casing section having a plurality of holes plugged with ceramic rupture discs, inserts or other frangible devices which can be ruptured by an acoustical or pressure wave from inside the well casing.
In the completion of oil and gas wells, it is a common practice to cement a casing string or liner in a wellbore and to perforate the casing string at a location adjacent to the oil or gas containing formation to open the formation into fluid communication with the inside of the casing string. To carry out this perforating procedure, numerous perforating devices have been developed which direct an explosive charge to penetrate the casing, the cement outside the casing and the formation.
In many instances in the completion and service of oil and gas wells, it is desirable to have a method and apparatus whereby perforations can be opened in the well casing string without penetrating the various layers of cement, resin-coated sand or other material located around the exterior of the casing string. Also, in some instances, it is desirable to isolate sections of the well casing such that the sections do not have cement or other materials around the exterior of the isolated section. That is, there is cement above and below a casing section but not around it, which leaves an open annulus between the casing and the wellbore and associated formation. It may further be desirable to penetrate such a section without the perforation penetrating the formation itself.
The present invention provides an apparatus and method for carrying out such procedures by utilizing a casing section which is plugged with ceramic discs, inserts or other frangible devices which can be ruptured by an acoustical or pressure wave within the well casing. In some embodiments, this pressure wave is produced by a small explosive charge detonated within the well casing. In others, the pressure wave is suddenly applied in the casing string, or acoustical wave generating devices are utilized.
The present invention includes an apparatus for opening perforations in a casing string by removing frangible rupture discs or other frangible devices in a casing string disposed in a wellbore and also relates to methods of perforating using this or similar apparatus.
The apparatus comprises a casing string positionable in the wellbore, the casing string itself comprising a casing section defining a plurality of holes through a wall thereof. The apparatus further comprises a rupturable plug means disposed in each of the holes in the casing section for rupturing in response to an impingement by a pressure wave, and a pressure wave generating device for generating the wave. The pressure wave generating device is separate from the casing string and positionable therein after the casing string is positioned in the wellbore. The rupturable plug means is preferably characterized by a disc or insert made of a glass ceramic material which will withstand differential pressures thereacross but will fracture in response to impingement by the pressure wave.
The apparatus further comprises retaining means for retaining the inserts in the holes prior to the rupture of the inserts. The retaining means may comprise a shoulder in each of the holes for preventing radially inward movement of the inserts. The retaining means may also comprise a retainer ring disposed in each of the holes for preventing radially outward movement of the inserts. In another embodiment, the retaining rings may comprise an adhesive disposed between the inserts and a portion of the casing string defining the holes. In an additional embodiment, the retaining rings may comprise a backup ring threadingly engaged with each of the holes for preventing radial outward movement of the inserts. In still another embodiment, the retaining rings may comprise a case threadingly engaged with each of the holes and defining an opening therein, wherein each of the inserts is disposed in one of the openings in a corresponding case.
The apparatus may further comprise a sealing means for sealing between the inserts and the casing string section. The sealing means may be characterized by a sealing element, such as an O-ring, or may include the adhesive previously described.
In one embodiment, the wave generating device is positionable in the casing string adjacent to the plug means on a length of fluid-filled tubing. The wave generating device may be a negative pulser, a coil tubing collar locator or a pressure pulse generator. The wave generating device may also be an acoustic wave generating device so that the pressure wave is an acoustic wave. In one embodiment, the acoustic wave generating device is an acoustical horn.
The invention also includes a method of opening perforations in a well casing without damaging areas outside the well casing. This method comprises the steps of providing a casing string in a wellbore wherein the casing string has a section defining a plurality of rupturably plugged holes therein, positioning a wave generating device in the casing string adjacent to the holes, and generating a pressure wave with the wave generating device such that the pressure wave impinges on the plugged holes and ruptures and unplugs the holes. The step of providing a casing string may comprise plugging the holes with a glass ceramic material which will rupture in response to impingement by the pressure wave.
The step of providing the casing string may also comprise filling the section of the casing string with a fluid, most often a liquid such as salt water, brine or a hydrocarbon liquid, for transmitting the pressure wave therethrough. In this embodiment, the method may further comprise the step of pressurizing the fluid in the casing string prior to the step of generating the pressure wave.
In a preferred embodiment, the step of positioning the wave generating device in the method comprises connecting the wave generating device to a length of coil tubing, and positioning the device in the well using the tubing. This may also comprise filling the tubing with fluid. The wave generating device may be a negative pulser, a coil tubing collar locator or a pressure pulse generator.
In another embodiment, the wave generating device may be an acoustic wave generating device, and the pressure wave is an acoustic wave. The acoustic wave generating device may include an acoustical horn. The acoustical horn may be positioned in fluid in the casing section. The horn may be adapted to generate an acoustical wave sufficient to place frangible material plugging the holes in a resonant state such that the frangible material shatters.
In other embodiments of the invention, the pressure wave may be generated by a mild explosive force. Thus, the apparatus may also be described as comprising a casing string positionable in the wellbore, the casing string itself comprising a casing section defining a plurality of holes through a wall thereof. The apparatus further comprises a rupturable plug means disposed in each of the holes in the casing section for rupturing in response to impact by the mild explosive force, and explosive means for generating the explosive force in the casing section adjacent to the holes. The explosive force fractures the rupturable plug means and thereby opens the holes so that an inner portion of the casing string is placed in communication with an outer portion thereof. The rupturable plug means is preferably characterized by a disc or insert made of a ceramic material which will withstand differential pressures thereacross but will fracture in response to impact by the explosive force.
The explosive means may be characterized by a length of det-cord disposed along the longitudinal center line of the casing section. The det-cord preferably comprises an explosive present in the amount of about forty grams per foot to about eighty grams per foot, but additional types of det-cord or other explosive means may also be suitable.
The apparatus may be further described as a method for opening perforations in a well casing comprising the steps of providing a casing string in the wellbore, wherein the casing string has a section defining a plurality of plugged holes therein, and detonating an explosive charge in the casing string adjacent to the holes and thereby unplugging the holes. The step of providing the casing string preferably comprises plugging the holes with a ceramic material which will rupture in response to detonation of the explosive charge.
This method of the invention may further comprise, prior to the step of detonating, a step of isolating the section of the casing string by placing material above and below the section of the casing string in a wellbore annulus defined between the casing string and the wellbore. In one embodiment, the step of placing comprises cementing the well annulus above and below the section of the plugged casing string section.
This method also comprises placing the explosive charge in the well casing in the form of a portion of det-cord. Preferably, the det-cord is placed on the longitudinal center line of the casing string.
Numerous objects and advantages of the invention will become apparent to those skilled in the art when the following detailed description of the preferred embodiment is read in conjunction with the drawings which illustrate such embodiment.